Question 49118
What is a function?  What does the function f(x)=x^2 - 7x + 10 really mean?

A function is just a formula for finding the answer anytime you know an input value.  In your situation, your formula says:
#1:  take the input number (called x for lack of a better name), and square it (this is what the x^2 means)
#2: take the input number, and mulitply it by 7  (this is 7x)
#3: take the number 10.
#4 take your answer to #1, subtract answer #2 from it, and then add answer #3 (the number 10)
Your answer is the answer to f(x).

That's what f(x) really means....

Therefore, what is f(0)?
Plug 0 in for x everywhere you see it in the formula f(x):
f(0) = 0^2 - 7(0) + 10 
f(0) = 0 - 0 + 10
f(0) = 10

What is f(5)?
Plug 5 in for x everywhere you see it in the formula f(x):
f(5) = (5)^2 - 7(5) + 10
f(5) = 25 - 35 + 10
f(5) = 0

What is f(-2)?
Plug -2 in for x everywhere you see it in the formula f(x):
f(-2) = (-2)^2 - 7(-2) + 10
f(-2) = 4 + 14 + 10
f(-2) = 28

OK? I hope this helps.
This idea also works for things like f(t), f(2a), and f(t+1).  Just plug in t, 2a, and t+1, and then simplify.  You won't get a number answer, but rather a formula as an answer.

Good luck...